Among the various video display systems available in the art, an optical projection system is known to be capable of providing high quality images in a large scale.
In FIG. 1, there is presented a schematic view of a prior art optical projection system 10 comprising a non-point white light source 11, a source lens 20, a source stopper 25 provided with a source aperture 26, an optical means 50 having a reflection surface 55, a field lens 60, a RGB pixel filter 40, a projection stopper 35 provided with a projection aperture 36, a projection lens 30, a projection screen 90 and an array 70 of M.times.N thin film actuated mirrors 71.
In such a system, a white light emanating from the non-point white light source 11 is focused along a first optical light path onto the source stopper 25 by the source lens 20, wherein the white light has a first, a second and a third primary light beams, each of the primary light beams being of one of the primary colors. The source stopper 25 is used for shaping the emanated white light from the non-point white light source 11 after it passes through the source lens 20 into a predetermined configuration by allowing a certain portion of the white light to pass through the source aperture 26 thereof. The transmitted white light from the source stopper 25 having the predetermined configuration travels onto the optical means 50, wherein the reflection surface 55 of the optical means 50 is in a facing relationship with the field lens 60 and the source stopper 25. The reflected white light from the reflection surface 55 of the optical means 50 diverges along a second optical light path and is collimated by the field lens 60, thereby being uniformly projected onto the RGB pixel filter 40, wherein the RGB pixel filter 40 is disposed between the field lens 60 and the array 70 of thin film actuated mirrors 71 and is in a facing relationship with the array 70 of thin film actuated mirrors 71. The RGB pixel filter 40 receives the collimated white light from the field lens 60, divides it into the first, the second and the third primary light beams, and transmits the primary light beams to the array 70 of the thin film actuated mirrors 71. Each of the thin film actuated mirrors 71 in the array 70 has a mirror 76, an actuator 72 and an active matrix 74, wherein the actuator 72 is made of a piezoelectric or an electrostrictive material which deforms in response to an electric signal applied thereto. Each of the thin film actuated mirrors 71 in the array 70 corresponds to one of the pixels to be displayed.
A third optical light path of the primary light beams reflected from each of the thin film actuated mirrors 71 in the array 70 is determined by the amount of deformation of the actuator 72 in each of the thin film actuated mirrors 71 in the array 70.
The primary light beams reflected from each of the thin film actuated mirrors 71 in the array 70 diverge along the third optical light path and are focused back to the projection stopper 35 by the field lens 60 after repassing through the RGB pixel filter 40. As the optical means 50 is not located on the third optical light path of the primary light beams, the primary light beams reflected from each of the thin film actuated mirrors 71 in the array 70 are focused back directly onto the projection stopper 35 by the field lens 60.
The primary light beams reflected from each of the undeflected thin film actuated mirrors 71 in the array 70 are focused back to the projection stopper 35 along the third optical light path by the field lens 60 after repassing through the RGB pixel filter 40 so that the primary light beams do not pass through the projection aperture 36 of the projection stopper 35. However, the primary light beams reflected from each of the deflected thin film actuated mirrors 71 in the array 70 are focused back to the projection stopper 35 along the third optical light path by the field lens 60 after repassing through the RGB pixel filter 40 so that the primary light beams pass through the projection aperture 36 of the projection Stopper 35, thereby modulating the intensity of the primary light beams. The primary light beams which pass through the projection aperture 36 of the projection stopper 35 travel to the projection lens 30 which projects the primary light beams transmitted from the projection aperture 36 on the projection screen 90, thereby displaying the image made up of M.times.N number of the pixels.
There are a number of problems associated with the optical projection system 10 described above. The first and foremost is a noise problem caused by the RGB pixel filter 40 reflecting a certain portion of the reflected white light from the reflection surface 55 of the optical means 50. Since the RGB pixel filter 40 is located directly on the second optical light path between the field lens 60 and the array 70 of the M.times.N thin film actuated mirrors 71, it may reflect the certain portion of the reflected white light from the reflection surface 55 of the optical means 50, and this will be reflected as the noise at the projection screen 90. Furthermore, repassing of the primary light beams reflected from each of the thin film actuated mirrors 71 in the array 70 through the RGB pixel filter 40 will reduce the intensity of the primary light beams, thereby reducing the overall optical efficiency of the system 10.